This application is the renewal application for our current R01 that funds the successful collaborative work of the Wernig and Sdhof laboratories. The goal of the previous funding period of this grant was to develop induced neuronal (iN) cell protocols to provide the proof-of-principle that cell biological phenotypes can be studied in human neurons carrying autism-associated mutations. We have accomplished this goal which allows us now to include iN cells into our portfolio of model systems to investigate synaptic molecules affected in and relevant for neuropsychiatric diseases. In this renewal application, we propose to now apply the technologies developed in the previous funding period and use it to actually study neuronal cell biology in human neurons. The goal of this renewal application is to understand the molecular functions of normal and mutant neuroligins including their sorting to specific synaptic specializations, the identification of their critical binding partners, their detiled role on synaptic modulation, and the mechanism of action of mutant forms of neuroligins found in autism. Neuroligins are critical synaptic cell-adhesion molecules localized as constitutive homo- or heterodimers at the post-synaptic membrane. Their large extracellular domain interacts with neurexins and potentially other presynaptic membrane proteins, their smaller intracellular domain with various postsynaptic scaffold proteins such as PSD-95. We will employ well-established and widely accepted mouse models to define functional neuroligin domains and interaction partners of normal and experimentally modified neuroligins. Most autism-associated neuroligin mutations are found in the X-chromosomal NLGN4 gene. The mouse Nlgn4, however, is not well conserved with a mere 51% sequence similarity on the protein level and is much lower expressed than Nlgn1-3, which questions the relevance of mouse studies for human disease. For this reason, we propose to utilize the human iN cell systems that we developed to investigate normal and mutant NLGN4 synaptic function. The combined Wernig and Sdhof laboratories provide the complementary expertise to make such experiments realistically feasible.